For an aircraft, the in-flight formation of ice on the external surface of the aircraft is undesirable. The ice destroys the smooth flow of air over the aircraft surface, increases drag and decreases the ability of an aerofoil to perform its intended function.
Also, built-up ice may impede the movement of a movable control surface such as a wing slat or flap. Ice which has built up on an engine air inlet may be suddenly shed in large chunks which are ingested into the engine and cause damage.
It is therefore common for aircraft, and particularly commercial aircraft, to incorporate an ice protection system. A commercial aircraft may use a system which involves bleeding hot air off from the engines, and the hot air is then ducted to the airframe components such as the leading edges of the wing and the tail which are prone to ice formation. More recently, electrically powered systems have been proposed, such as in EP-A-1,757,519 (GKN Aerospace) which discloses a wing slat having a nose skin which incorporates an electro-thermal heater blanket or mat. The heater mat is bonded to the rear surface of a metallic erosion shield which comprises the forwardly-facing external surface of the nose skin.
The heater mat is of the SPRAYMAT® (trade mark) type and is a laminated product comprising dielectric layers made of preimpregnated glass fibre cloth and a heater element formed by flame spraying a metal layer onto one of the dielectric layers. The SPRAYMAT has a long history from its original development in the 1950s by D. Napier & Sons Limited (see their GB-833,675 relating to electrical de-icing or anti-icing apparatus for an aircraft) through to its subsequent use by GKN Aerospace.
Modern designs of heater mat are disclosed in GB-A-2,477,336 and GB-A-2,477,337 and GB-A-2,477,338 and GB-A-2,477,339 and GB-A-2,477,340 which are all in the name of GKN Aerospace.
In order to know when to initiate operation of a heater mat, an aircraft is currently provided with an ice-detection sensor which is remote from the location of the heater mat. For example, the ice-detection sensor may be incorporated into a stub-shaped probe which is mounted on the fuselage adjacent to the cockpit and which projects into the airstream.
The ice-detection sensor may be an optical sensor such as the forward-looking sensor which is described in WO-2004/110865 and which has a fibre optic light emitter at the centre of an array of fibre optic sensor elements (for example, a linear array of six sensor elements). Accreted ice causes the emitted light to be diffusely scattered and reflected back into the sensor elements, and the characteristics of the output signals from the sensor elements enable control electronics to determine the thickness of the accreted ice and the type of the accreted ice (e.g. glaze ice, rime ice, or mixed ice comprising glaze ice and rime ice).
The result of the ice detection is provided as advice or information to the pilot in the cockpit using a visual and/or audible annunciator, and the pilot makes a decision as to whether to activate the heater mats of the ice-protection system.
The probe which incorporates the optical ice sensor has an aerodynamic profile which is different to the aerodynamic profile of the airframe component in which the heater mat is incorporated. Also, the probe is remote from the airframe component. Thus icing of the airframe component has to be inferred from detected icing of the probe.
The pilot, based on the output of the probe, may activate the heater mats when de-icing of the airframe component is not needed (which would waste power by unnecessarily operating the heater mats) or may fail to activate the heater mats when the airframe component has accreted ice which is causing a problem although the output from the probe does not indicate to the pilot that there is an icing problem.
When the pilot does activate the heater mats (for example heater mats in the wing slats and in the leading edges of the tailplane and the tail fin and the engine nacelles) the heater mats are operated in a predetermined sequence involving all of the heater mats. This means that power may be wasted heating some parts of the wing slats, tailplane, tail fin and engine nacelles which do not currently need to be heated because the parts in question currently are not covered with accreted ice or with ice which is thick enough to merit de-icing. Power is wastefully used to heat these parts when it would be more efficiently used to heat only those parts which are covered with accreted ice of sufficient thickness to merit de-icing.
It would be desirable to provide an improved ice protection system and method of operation thereof.